Exhaust recirculation

ABSTRACT

Automobile engine exhaust gases are recycled through a recycling duct having a restricted upstream end opening into the exhaust system and first and second restricted downstream ends opening into the carburetor inlet induction conduit downstream and upstream respectively of the throttle valve. The restrictions for the recycling duct are dimensioned with respect to each other so that when the throttle valve is at its closed or idle position, a major quantity of the air required to support engine combustion at idle will bypass the throttle valve and flow through the portion of the recycling duct from the second downstream end to the first. The second restricted downstream end opens into the induction conduit at the usual fuel metering venturi restriction and is restricted appreciably less than the first downstream end, such that when the throttle valve is opened from the idle position, the first downstream end will have only a minor influence on the pressure in the recycling duct and the exhaust gas flow thereinto from the exhaust conduit via the restricted upstream end will be substantially a function of the pressure differential between the exhaust pressure at the upstream end and the venturi pressure at the second downstream end. The induction conduit pressure immediately upstream of the upstream leading edge of the blade of the throttle valve, when the latter is at its idle position, is employed to operate a valve to close the recycling duct completely at a location upstream of the two downstream ends during both idle and wide open throttle operation of the engine.

United States Patent [191 Sarto EXHAUST RECIRCULATION Jorma O. Sarto, Orchard Lake, Mich.

[73] Assignee: Chrysler Corporation, Highland Park, Mich.

221 Filed: Sept. 21, 1972 21 Appl. No.: 291,115

[75] Inventor:

Primary Examiner-Manuel A. Antonakas Assistant Examiner-Daniel J. OConnor Attorney, Agent, or Firm-Talburtt & Baldwin [57] ABSTRACT Automobile engine exhaust gases are recycled through a recycling duct having a restricted upstream end opening into the exhaust system and first and second restricted downstream ends opening into the carbure- Aug. 6, 1974 tor inlet induction conduit downstream and upstream respectively of the throttle valve. The restrictions for the recycling duct are dimensioned with respect to each other so that when the throttle valve is at its closed or idle position, a major quantity of the air required to support engine combustion at idle will bypass the throttle valve and flow through the portion of the recycling duct from the second downstream end to the first. The second restricted downstream end opens into the induction conduit at the usual fuel metering venturi restriction and is restricted appreciably less than the first downstream end, such that when the throttle valve is opened from the idle position, the first downstream end will have only a minor influence on the pressure in the recycling duct and the exhaust gas fiow thereinto from the exhaust conduit via the restricted upstream end will be substantially a function of the pressure differential between the exhaust pressure at the upstream end and the venturi pressure at the second downstream end. The induction conduit pressure immediately upstream of the upstream leading edge of the blade of the throttle valve, when the latter is at its idle position, is employed to operate a valveto close the recycling duct completely at a location upstream of the two downstream ends during both idle and wide open throttle operation of the engme.

17 Claims, 2 Drawing Figures 1 EXHAUST RECIRCULATION BACKGROUND AND SUMMARY OF THE INVENTION In the prior art, numerous systems have been devised to recycle exhaust gas into the fuel-air induction system of an automobile engine for the purposes of pre-heating and vaporizing the incoming air-fuel mixture to facilitate its complete combustion in the combustion zone,

for re-using the unignited or partially burned portions of the fuel which would otherwise pass out the exhaust pipe and into the atmosphere, and for reducing the ox ides of nitrogen emitted from the exhaust system into the atmosphere. It has been found that approximately l5percent exhaust gas recycling is required at moderate engine loads to substantially reduce the nitrogen oxide content of the exhaust gases.

Although the prior art structures have had the desired effect of reducing the content of nitrogen oxides in the exhaust, these structures have been undesirable from the standpoints of both cost and operating efficiency and have been complicated by the desirability of reducing the recycling during conditions of both engine idling when nitrogen oxide emission is a minor problem and wide open throttle when maximum power is required, while progressively increasing the recycling of exhaust gases with increasing engine speed during cruising condition or with increasing engine load at part open throttle.

In the usual gasoline or hydrocarbon fuel type engine, fuel combustion can take place at about l,200F. The formation of nitrogen oxides does not become particularly objectionable until the combustion temperature exceeds about 2,200F., but the usual engine combustion temperature which increases with engine load rises to about 2,500F. It is known that the recycling of at least l/th and not more than one-quarter of the total exhaust gases through the engine, depending on the load or power demand, will reduce the combustion temperature to less than 2,200F. The desired result is usually obtained with the ordinary engine upon the recycling of about 15 percent of the total exhaust gases during partially open throttle as aforesaid.

An important object of this invention is to provide improved means comprising a minimum of moving parts for recirculating a portion of the combustion products from the exhaust system to the inlet system of an automobile engine to overcome or avoid the problems and deficiencies of the prior art, as well as to achieve a number of important results including preheating and improved mixing and carburetion of the fuel-air mixture in the inlet header and the reduction of nitrogen oxides in the exhaust.

Another and more specific object is to provide such an exhaust recycling system comprising an exhaust recycling duct having an upstream restricted end opening into the exhaust system to receive exhaust gases and having first and second downstream restricted ends opening into the carburetor induction conduit downstream and upstream respectively of the throttle valve, the restriction at the first downstream end, i.e. downstream of the throttle valve, being appreciably greater than the restriction at the second downstream end which opens into the inlet induction conduit upstream of the throttle valve at the region of the fuel metering venturi restriction, whereby the'influence of thefirst downstream end on the pressure within the recycling duct is minor when the throttle valve is opened from its idle position. Thus, the exhaust recycling flow will be substantially a function of the pressure differential between the exhaust system at the upstream end of the recycling duct and the venturi pressure at the second downstream end, which pressure differential will be a function of the inlet air flow. The restriction for the upstream end of the recycling duct is comparable to or slightly greater than the restriction at the second downstream end and is dimensioned to obtain the desired exhaust gas recycling approximating 15 percent of the total exhaust flow, which flow will also be a function of the aforesaid pressure differential. By virtue of the slight restriction for the second downstream end as compared to the first downstream end, when the throttle valve is at its idle position and both the venturi pressure and exhaust pressure are almost atmospheric, very little pressure differential will exist across the upstream opening and exhaust gas flow through the latter opening from the exhaust system will be nominal. Also during idle operation, the pressure within the inlet induction conduit at the first restricted end downstream of the throttle valve will be low and a bypass flow of inlet air around the throttle valve will flow through the portion of the recycling duct from the second to the first downstream end. The restriction for the first downstream end is dimensioned so that the idle bypass flow will amount to between approximately 50 percent and percent of the air desired to support the idle combustion. The remaining idle air will be supplied by leakage around the throttle valve and with the idle fuel supply in accordance with customary practice.

By virtue of the foregoing, communication may exist at all times between the exhaust and inlet systems but recycling of the hot exhaust gases will be reduced during idle operation. Also the high temperature of the exhaust at wide open throttle tends to reduce the exhaust density as compared to the lower temperatures during moderate acceleration, so that the effective exhaust recycling at wide open throttle is less than it would be otherwise.

Within the range from idle to light or moderate load conditions, the total fluid flow through recycling restrictions or orifices of the type comprising the present invention increases at any given engine speed with increasing engine load. For example in a conventional automobile engine, the pressure downstream of the throttle varies roughly in the neighborhood of from one half atmosphere during idling to approximately one atmosphere at wide open throttle, while while exhaust pressure simultaneously varies roughly from one to two atmospheres. These factors compensate for the increasing combustion temperature with increasing load and result in a desirable increase in the effectiveness of the exhaust recycling through the recycling restrictions with increasing load or acceleration.

As the pressure differential between the inlet venturi restriction and the exhaust system increases with increasing load, the effective resistance of the fixed restrictions to the exhaust recycling flow increases because the flow rate through these restrictions varies approximately as the square root of the pressure differential. Thus, at wide open throttle, the proportion of the total exhaust gases that is recycled is somewhat less than the proportion recycled at partially open throttle. This factor also is as desired because the customary exvent overheating during the combustion process and reduce the formation of nitrogen oxides to the tolerable level.

Another object is to provide valve means cooperable with an exhaust recycling system of the above character and responsive to opening of the throttle valve for positively closing the recycling duct during idle and wide open throttle conditions, thereby to prevent all exhaust recycling during such conditions.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

FIG. 1 is a schematic fragmentary cross sectional view through an automobile engine induction system showing a system embodying the present invention for recycling exhaust gases.

FIG. 2 is a view similar to FIG. 1, showing a modification.

DESCRIPTION OF A PREFERRED EMBODIMENT An application of the present invention is illustrated by way of example with an automobile engine 9 having a carburetor 10 providing the inlet fuel-air induction conduit 11, which comprises the upstream portion of an inlet header 12 for supplying a combustible fuel and air mixture to the engine cylinders. The carburetor 10 may comprise any conventional type which has the usual air inlet at the upstream end of the induction conduit 11, the usual fuel metering system including venturi restriction 13 and nozzles or jets (not shown) for supplying idle and operating fuel to the conduit 11 during various operating conditions and for enriching the fuel supply during acceleration and wide open throttle. An example of such a carburetor is illustrated by way of example in Ball U.S. Pat. No. 2,966,344 and the foregoing conventional features disclosed in the latter patent are incorporated herein by reference.

The downstream portion of the induction conduit 11 comprises the customary throttle body containing the conventional butterfly type throttle valve 14 pivotal at 15. The inlet fuelair mixture is conducted via the header or manifold 12 to the engine cylinders. After combustion of the fuel-air mixture in the engine cylinders, the exhaust gases are discharged through a muffler to the atmosphere.

Usually left and right exhaust manifolds are connected by an exhaust crossover conduit 16 which conducts the hot exhaust gases into heat exchange relationship with a portion 17 of the wall of the inlet header 12. The wall portion 17 extends transversely to the direction of flow of the inlet mixture and is commonly referred to as the hot spot which preheats the inlet mixture and enhances vaporization and mixing of liquid fuel droplets.

The structure described thus far may also be conventional, as illustrated in Sarto U.S. Pat. No. 3,646,923, incorporated herein by reference.

Associated with the throttle valve 14 and extending through the hot spot 17 is a portion of a restricted exhaust recycling duct 18 which extends directly from the exhaust cross-over conduit 16 and has an upstream orifree or restriction 19 at its lower end opening into the conduit 16 to receive exhaust gases. The duct 18 discharges through a first downstream restriction or orifice 20 into the header 12 directly below valve 14 and in opposition to the inlet fuel-air flow to warm the latter and to facilitate mixing and vaporization of the fuel within the inlet air.

In order to inhibit exhaust flow through restricted orifice 19 during idle operation of the engine when the throttle valve 14 is at the idle position shown by solid lines, the nearly atmospheric inlet pressure above the valve 14 is communicated to the upper end of duct 18 via a second downstream orifice or restriction 21. The upper portion of duct 18 extends within the sidewall of the conduit 11 from port 19 to port 21, which latter opens into conduit 11 upstream of the valve 14 at the diffuser region of the venturi 13. The cross sectional area of restriction 21 is preferably 4 to 10 times as large as the cross sectional area of port 20 and is comparable in area or slightly larger than port 19. In consequence during idle operation, the approximate atmospheric pressure in conduit 11 upstream of throttle 14 will be transmitted substantially to conduit 18 and only a comparatively small pressure differential wall exist across restriction 19 because the exhaust pressure in passage 16 will only be slightly greater than atmospheric during idle. The actual pressure differential will be determined by the sizes of the orifices 19, 20 and 21. The low pressure downstream of throttle 14 during idle will induce a bypass flow of idle combustion supporting air around throttle 14 via ports 21 and 20, the latter being dimensioned to pass between approximately 50 percent and percent of the air required to support combustion of the idle fuel. The remaining idle air will be supplied by leakage around the edges of throttle valve 14 and via the customary idle fuel port in a partially atomized fuelair mixture. The orifice 21 may be comparable in size to orifice 19, or somewhat larger, and is determined so that the exhaust recycling at idle will be reduced to a value in the neighborhood of 5 percent of the total exhaust and preferably less. The exhaust flow through port 20 tends to maintain the temperature of the hot spot 17 during idle and also tends to heat the latter more rapidly to an effective operating temperature after a cold start.

As the throttle valve 14 opens from the idle position shown, the pressure differential across the valve 14 will fall rapidly and the bypass air flow around valve 14 will cease. Likewise the venturi pressure at port 21, which is an inverse function of the air flow in conduit 11, will decrease rapidly. The exhaust recycling flow through port 19 will then be determined primarily by the pressure differential between the venturi pressure at port 21 and the exhaust pressure at port 19, which as aforesaid will be a function of the air flow in conduit 11 and will be proportioned between orifices 20 and 21 at the two downstream ends of recycling duct 18 in accordance with their relative restrictions to effect a total exhaust recycling flow as aforesaid in the neighborhood of IS percent of the total exhaust flow. As the throttle 14 progressively opens, the venturi pressure at restriction 21 progressively decreases with increasing air flow and the pressure at the opening of restriction 20 into conduit 11 increases. The proportion of exhaust gas flow into conduit 11 through the downstream restriction 21 will thus gradually increase with respect to the exhaust gas flow through downstream restriction 20. However, orifice 19 is sufficiently large so that the pressure in recycling duct 18 will always be greater than the pressure in conduit 11 at restriction 20, resulting in some exhaust gas recycling through restriction 20 at all times.

At wide open throttle, the upwardly directed orifice 20 will not be shielded by the throttle valve from the full blast of the inlet flow, so that the exhaust recycling through restriction 20 will be hampered by pitot action. This factor, in cooperation with the aforesaid flow relationship proportional to the square root of the pressure differential across orifice 19 and the temperaturedensity relationship of the high temperature exhaust gas at wide open throttle, will effect a reduction in the proportion of exhaust recycling through duct 18 at wide open throttle, compared to the total exhaust flow, even through the total exhaust recycling will increase steadily as the throttle approaches the wide open condition.

FIG. 2 illustrates a modification of the present invention substantially the same in concept and operation as described above, except that a pressure operated valve 22 secured to the engine 9 'is employed to prevent exhaust gas recycling at idle and wide and open throttle conditions. Corresponding parts are numbered the same in both views. Valve 22 comprises a chamber 23 defined in part by a flexible diaphragm 24 and in communication via duct 25 with a port 26 which opens into the conduit 11 adjacent and upstream of the leading upstream edge of the blade of throttle valve 14. Port 26 may comprise the usual vacuum spark advance port in communication with a pressure operated spark advance mechanism via duct 27.

A valve rod 28 secured to diaphragm 24 extends slidably and in sealing relationship through the wall of duct 18 and terminates in a valve disc 29 adopted to seat at an annular valve seat 30 comprising a coaxial portion of duct 18 downstream of restriction or orifice 19. A coil spring 31 within chamber 23 normally urges diaphragm 24 and rod 28 downwardly to seat valve 29 at a closed position against seat 30 whenever the pressure communicated to chamber 23 via port 26 attains or exceeds a predetermined value corresponding to idle or wide open throttle operation.

Accordingly, when throttle valve 14 is at the idle position illustrated in solid lines, FIG. 2, the nearly atmospheric pressure at port 26 will enable spring 31 to close valve 29 and positively prevent exhaust gas recycling. The bypass flow of idle combustion supporting air around throttle 14 will be conducted from conduit 11 through restriction 21, a downstream portion of duct 18, and then back into conduit 11 via restriction 20 as described above. Upon opening of throttle 14 from the closed or idle position to the dotted position shown, port 26 will be located at the low pressure downstream side of valve 14 and the reduced pressure at port 26 will cause diaphragm 24 to move upward against the force of spring 31 and unseat valve 29 to the dotted position, whereby communication between upstream restriction 19 and the two downstream restrictions 20 and 21 is established and exhaust recycling will take place as already described. Upon continued opening of throttle valve 14 to near its wide open position, the pressure at port 26 will gradually increase, again to cause closing of valve 29 to stop exhaust recycling. A reverse throttle bypass flow of the fuel-air mixture may flow from port 20 to port 21, but such flow is not objectionable. In FIG. 2 orifice 19 does not pass through hot spot 17 in heat exchange relationship, nor does it discharge into inlet or conduit 11 in opposition to the downward flow of the inlet fuel-air mixture. These latter features are optional and may be retained by suitably locating restriction 20 as in FIG. 1 and by locating port 19 in the wall portion 18a of duct 18, by way of example.

I claim:

1. In an internal combustion engine,

an inlet header for conducting a combustible fuel air mixture into said engine and having I. a venturi restriction therein and 2. a throttle valve therein downstream of said venturi restriction and movable to idle and open positions for controlling fluid flow therethrough, an exhaust header for discharging combustion products from said engine, and means for recycling exhaust gases from said exhaust header to said inlet header as a function of the rate of fluid flow through said inlet header to inhibit the formation of nitrogen oxides during combustion comprising recycling duct means having 1. a restricted upstream opening communicating with said exhaust header to receive exhaust gases, and

2. first and second restricted downstream openings communicating with said inlet header in parallelism with each other at locations downstream and upstream respectively of said throttle valve to discharge said exhaust gases into said inlet header,

3. the sizes of the restrictions for said openings being in predetermined relationship with respect to each other and said second downstream opening communicating with said inlet header at a restricted region thereof comprising said venturi restriction.

2. In the combination according to claim 1, said ex- 0 haust header having a wall portion extending transversely to the flow of said fuel-air mixture in said inlet header to provide a hot wall for impingement of said mixture thereagainst, said first downstream opening extending through said hot wall in heat transfer relationship.

3. In the combination according to claim 2, said first downstream opening being directed in opposition to the flow of said fuel-air mixture at the region of said hot wall.

4. In the combination according to claim 1, said first downstream opening being smaller than said upstream opening and being dimensioned to provide a flow of inlet air bypassing said throttle amounting to the major portion of the idle combustion supporting air when said throttle valve is at its idle position.

5. In the combination according to claim 4, said second downstream opening being between approximately 4 and 10 times as large as said first downstream openmg.

6. In the combination according to claim 5, said exhaust header having a wall portion extending transversely to the flow of said fuel-air mixture in said inlet header to provide a hot wall for impingement of said mixture thereagainst, said first downstream opening extending through said hot wall in heat transfer relationship.

7. In the combination according to claim 1, said second downstream opening being between approximately 4 and 10 times as large as said first downstream openmg.

8. In the combination according to claim 1, the cross sectional areas of the restrictions for each of said upstream and second downstream openings being sufficiently large with respect to the cross sectional area of said first downstream opening to maintain a flow of exhaust gases through said upstream opening determined approximately by the pressure differential between the exhaust pressure in said exhaust header and the venturi pressure in said inlet header at the region of said second downstream opening when said throttle valve is opened a predetermined extent from said idle position.

9. In the combination according to claim 8, the cross sectional area of said second downstream opening being several times the cross sectional area of said first downstream opening.

10. In the combination according to claim 9, said first downstream opening being dimensioned to pass between approximately 50 percent and 80 percent of the air required for idle combustion when said throttle valve is at its idle position.

11. In the combination according to claim 10, said second downstream opening between located at the diffuser region of said venturi.

12. In the combination according to claim 1, said first restricted downstream opening of said recycling duct means into said inlet header comprising a throttle bypass system for idle air during idle operation of said engine, and valve means responsive to said idle operation for closing said recycling duct means at a location between said upstream restriction and said downstream restriction.

13. In the combination according to claim 12, said first downstream opening being smaller than said upstream opening and being dimensioned to provide a flow of inlet air bypassing said throttle amounting to the major portion of the idle combustion supporting air when said throttle valve is at its idle position.

14. In the combination according to claim 13, said second downstream opening being between approximately 4 and 10 times as large as said first downstream opening.

15. In the combination according to claim 12, the cross sectional area of said second downstream opening being several times larger than the cross sectional area of said first downstream opening, and means responsive to wide open throttle operation for operating said valve means to close said recycling duct means at said location.

16. In the combination according to claim 15, said valve means and means responsive to said wide open throttle operation comprising a recycling valve movable between open and closed positions to open or close respectively the communication between said upstream opening and said downstream openings at said location, and means responsive to the pressure in said inlet header upstream of said throttle valve when the latter is at its idle position for moving said recycling valve to its closed position when the last named pressure exceeds a predetermined pressure and for moving said recycling valve to its open position when said last named pressure is not greater than the first named predetermined pressure.

17. In the combination according to claim 16, said throttle valve comprising a blade type valve having an upstream leading edge, said means responsive to the pressure in said inlet header at a location upstream of said throttle valve comprising pressure responsive means in communication with said inlet header at a location adjacent and upstream of said leading edge when said throttle valve is at its idle position, said location being downstream of said leading edge when said throttle valve is opened to a predetermined position. 

1. In an internal combustion engine, an inlet header for conducting a combustible fuel-air mixture into said engine and having
 1. a venturi restriction therein and
 2. a throttle valve therein downstream of said vEnturi restriction and movable to idle and open positions for controlling fluid flow therethrough, an exhaust header for discharging combustion products from said engine, and means for recycling exhaust gases from said exhaust header to said inlet header as a function of the rate of fluid flow through said inlet header to inhibit the formation of nitrogen oxides during combustion comprising recycling duct means having
 1. a restricted upstream opening communicating with said exhaust header to receive exhaust gases, and
 2. first and second restricted downstream openings communicating with said inlet header in parallelism with each other at locations downstream and upstream respectively of said throttle valve to discharge said exhaust gases into said inlet header,
 3. the sizes of the restrictions for said openings being in predetermined relationship with respect to each other and said second downstream opening communicating with said inlet header at a restricted region thereof comprising said venturi restriction.
 2. first and second restricted downstream openings communicating with said inlet header in parallelism with each other at locations downstream and upstream respectively of said throttle valve to discharge said exhaust gases into said inlet header,
 2. a throttle valve therein downstream of said vEnturi restriction and movable to idle and open positions for controlling fluid flow therethrough, an exhaust header for discharging combustion products from said engine, and means for recycling exhaust gases from said exhaust header to said inlet header as a function of the rate of fluid flow through said inlet header to inhibit the formation of nitrogen oxides during combustion comprising recycling duct means having
 2. In the combination according to claim 1, said exhaust header having a wall portion extending transversely to the flow of said fuel-air mixture in said inlet header to provide a hot wall for impingement of said mixture thereagainst, said first downstream opening extending through said hot wall in heat transfer relationship.
 3. the sizes of the restrictions for said openings being in predetermined relationship with respect to each other and said second downstream opening communicating with said inlet header at a restricted region thereof comprising said venturi restriction.
 3. In the combination according to claim 2, said first downstream opening being directed in opposition to the flow of said fuel-air mixture at the region of said hot wall.
 4. In the combination according to claim 1, said first downstream opening being smaller than said upstream opening and being dimensioned to provide a flow of inlet air bypassing said throttle amounting to the major portion of the idle combustion supporting air when said throttle valve is at its idle position.
 5. In the combination according to claim 4, said second downstream opening being between approximately 4 and 10 times as large as said first downstream opening.
 6. In the combination according to claim 5, said exhaust header having a wall portion extending transversely to the flow of said fuel-air mixture in said inlet header to provide a hot wall for impingement of said mixture thereagainst, said first downstream opening extending through said hot wall in heat transfer relationship.
 7. In the combination according to claim 1, said second downstream opening being between approximately 4 and 10 times as large as said first downstream opening.
 8. In the combination according to claim 1, the cross sectional areas of the restrictions for each of said upstream and second downstream openings being sufficiently large with respect to the cross sectional area of said first downstream opening to maintain a flow of exhaust gases through said upstream opening determined approximately by the pressure differential between the exhaust pressure in said exhaust header and the venturi pressure in said inlet header at the region of said second downstream opening when said throttle valve is opened a predetermined extent from said idle position.
 9. In the combination according to claim 8, the cross sectional area of said second downstream opening being several times the cross sectional area of said first downstream opening.
 10. In the combination according to claim 9, said first downstream opening being dimensioned to pass between approximately 50 percent and 80 percent of the air required for idle combustion when said throttle valve is at its idle position.
 11. In the combination according to claim 10, said second downstream opening between located at the diffuser region of said venturi.
 12. In the combination according to claim 1, said first restricted downstream opening of said recycling duct means into said inlet header comprising a throttle bypass system for idle air during idle operation of said engine, and valve means responsive to said idle operation for closing said recycling duct means at a location between Said upstream restriction and said downstream restriction.
 13. In the combination according to claim 12, said first downstream opening being smaller than said upstream opening and being dimensioned to provide a flow of inlet air bypassing said throttle amounting to the major portion of the idle combustion supporting air when said throttle valve is at its idle position.
 14. In the combination according to claim 13, said second downstream opening being between approximately 4 and 10 times as large as said first downstream opening.
 15. In the combination according to claim 12, the cross sectional area of said second downstream opening being several times larger than the cross sectional area of said first downstream opening, and means responsive to wide open throttle operation for operating said valve means to close said recycling duct means at said location.
 16. In the combination according to claim 15, said valve means and means responsive to said wide open throttle operation comprising a recycling valve movable between open and closed positions to open or close respectively the communication between said upstream opening and said downstream openings at said location, and means responsive to the pressure in said inlet header upstream of said throttle valve when the latter is at its idle position for moving said recycling valve to its closed position when the last named pressure exceeds a predetermined pressure and for moving said recycling valve to its open position when said last named pressure is not greater than the first named predetermined pressure.
 17. In the combination according to claim 16, said throttle valve comprising a blade type valve having an upstream leading edge, said means responsive to the pressure in said inlet header at a location upstream of said throttle valve comprising pressure responsive means in communication with said inlet header at a location adjacent and upstream of said leading edge when said throttle valve is at its idle position, said location being downstream of said leading edge when said throttle valve is opened to a predetermined position. 